His quest to win the Republican presidential nomination has always resembled a detailed, methodical business plan. Mr. Romney, who spent much of his life fixing troubled corporations, must now decide whether steps are necessary to repair his lethargic candidacy.

Mr. Romney had hoped that a string of Super Tuesday victories in contests from Vermont to Alaska would effectively bring the Republican race to a close.

But he found himself winning over Rick Santorum by only the slimmest of margins with almost all the votes counted in Ohio, the most coveted primary of the night, while losing other contests across the South.

The Republican race was always destined to plod on, considering that none of the candidates have reached even half of the 1,144 delegates needed to win the nomination. But the campaign is suddenly bracing for new questions about Mr. Romney’s ability to piece together a coalition needed to move closer to a general election fight with President Obama.

Mr. Romney had hoped a commanding victory over Mr. Santorum in Ohio would add another Midwestern battleground state to his tally and provide new latitude to begin directly engaging Mr. Obama. The heart of his case to his fellow Republicans has always been that he is not only the most electable candidate in the field, but also the best prepared and most suited for what is sure to be a brutal general election.

But the outcome of the contests on Tuesday, while allowing him to amass more delegates than any of his rivals, did little to resolve the questions about his ability to connect with voters, especially conservatives.

With the general election exactly eight months away, Republican leaders have increasingly argued that the time had come to move beyond the party’s messy intramural fight. They assumed that Mr. Romney’s strong financial advantages and muscular campaign organization would make that happen on Tuesday night.

Even the president anticipated a shift in the campaign. He staged his first White House news conference of the year hours before the votes started coming in to assail the Republican field for what he called irresponsible talk about war with Iran. He declined to answer questions about specific candidates, but flashed a smile when asked directly about Mr. Romney.

“Good luck tonight,” Mr. Obama said. “Really.”

It was an air of bravado from a man who endured his own long nominating fight four years ago. The story of Mr. Obama’s winding path to the White House is frequently offered as a testimonial by the Romney campaign when faced with questions about potential damage from a protracted and bitter primary season.

Supporters of Mr. Romney worry that he has been politically bruised by the right-leaning shift of the Republican contests, but it is far from certain that any damage will be lasting. Republicans may be deeply divided now, but defeating Mr. Obama is a powerful unifying force in the party.

Photo

Rick Santorum with his 93-year-old mother, Catherine, at an appearance at Steubenville High School after the Ohio primary.Credit
Ozier Muhammad/The New York Times

“I’d love for this to be over sooner rather later so we can focus on the president’s record,” Gov. Bob McDonnell of Virginia, a Romney supporter, said in an interview on Tuesday night. “But however long it takes, the desire to replace President Obama will motivate conservatives and libertarians in such a significant way.”

For Mr. Romney, the Super Tuesday scorecard was decidedly mixed.

He seized upon his wins in Idaho, Massachusetts, Vermont and Virginia — two of which are reliably Democratic states in presidential elections. But Mr. Santorum pointed to victories in North Dakota, Oklahoma and Tennessee as evidence that many Republicans were still looking for a conservative alternative. And Newt Gingrich held up a sweep of his home state of Georgia as proof of his own revival.

Mr. Romney built his victories Tuesday among a coalition of voters who say electability is their chief concern, as well as among older, better-educated and higher-income voters. The critical set of white, working-class voters divided their loyalties between Mr. Romney and Mr. Santorum.

But when voters in Ohio and Tennessee were asked which candidate best understood the problems of average Americans, according to exit polls, Mr. Romney fell short of Mr. Santorum.

The lineup of Super Tuesday states never looked particularly friendly to Mr. Romney. He campaigned in Georgia, North Dakota and Tennessee, hoping to win at least a share of the delegates, but he fought hard to earn a convincing win in Ohio. He fared poorly with very conservative voters and evangelical Christians, according to interviews with voters as they left the polls, and also struggled among voters who said it mattered a great deal that a candidate shared their religious beliefs.

Mr. Romney, whose candidacy has largely been built around a strategy of letting his Republican rivals fade away, is again facing the challenge of trying to improve his ability to connect with voters and excite them. The next string of contests, including Mississippi, Alabama, Kansas and Missouri, may also be challenging states in which to build momentum.

“I’m not going to let you down,” Mr. Romney told supporters gathered in a half-filled hotel ballroom in Boston. “I’m going to get this nomination.”

A significant share of advantages in the Republican nominating fight remains on Mr. Romney’s side. He has more money at his disposal than any of his rivals, particularly if he decides to invest his own money into the campaign, as he did during his first presidential run four years ago.

Even with a narrow win in Ohio, the scoreboard for Mr. Romney already includes victories in a succession of general election battlegrounds: New Hampshire, Florida, Nevada and Michigan. His successes have been more tactical than triumphant, but the delegates are worth the same. And Mr. Romney emerged from Super Tuesday with a strong mathematical advantage that cannot be overlooked.

But one of the best opportunities for Mr. Romney to accelerate his march to the nomination was to convincingly defeat Mr. Santorum in Ohio. The outcome was not clear by the time Mr. Romney took the stage to deliver his speech late Tuesday night.

“This is a process of gathering enough delegates to become the nominee,” Mr. Romney said. “I think we’re on the track to have that happen.”

He offered no predictions of when.

A version of this news analysis appears in print on March 7, 2012, on page A1 of the New York edition with the headline: With No Knockout Punch, a Bruising Battle Plods On. Order Reprints|Today's Paper|Subscribe